Friday, August 1, 2008

LRVs Without Overhead Wires?

Bombardier's "MITRAC Energy Saver" consists of an supercapacitor bank and associated electronic. Its primary use is to recover energy from braking, but Bombardier claims that 1 km of acceleration is possible.

The weakness of this technology is its low voltage. According to the product information, it runs at 750 volts. To get, say, 10 km of acceleration, the voltage has to increase to 7,500 volts. It is unclear from Bombadier's website how much charge is stored. Still, the supercapacitors have to have about 10 times greater voltage, or be 1/10th of the size of the current ones to store this much charge.

Thus, we can conclude that this technology is unripe.

So, what does this have to do with anything? I've been following the talk about light rail lines in Washington, DC. It seems that the Congress has long forbidden overhead wires on streets. Getting power from a third rail using either the old conduit track technology or the new technology in Bordeaux is impractical. So, the way to get streetcars running is to charge them at stations. Right now, this appears impractical. But, improvements in battery and supercapacitor technologies may make this feasible within 5 years. So, all of you who like to dream up light rail systems should think in terms of having chargers at terminals and stations, with Shanghai-like charging "umbrellas".

1 comment:

njh said...

Energy stored in a capacitor is CV^2/2, so to gain 10 times the energy you only need to raise the voltage by sqrt(10) = 3.2 times (or increase the capacitance by a factor of 10).

In any case it's irrelevant, because the stored energy goes as the mass or volume of the capacitors which ever route you take.

Personally I have no issue with overhead wires, and prefer that to particulates from rubber and diesel. 1km seems a pretty reasonable stretch between 'drinks' to me anyway.